Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).
An orthogonal FDM (OFDM) technique distributes the data over a large number of sub-carriers that are orthogonal to one another. The orthogonal property of the sub-carriers means that the sidebands of individual sub-carriers may overlap and the signals are still received without adjacent sub-carrier interference. The benefits of OFDM include high spectral efficiency, resiliency to RF interference, and lower multipath distortion. This is useful because in a typical terrestrial wireless communications implementation there are multipath channels (i.e., the transmitted signal arrives at the receiver using various paths of different length and different fading characteristics).
OFDM has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. For example, OFDM has been selected as the multiple access scheme by several standard organizations, including IEEE 802.11, IEEE 802.16, DVB-T (digital video broadcast-terrestrial), and DVB-H (handheld). OFDM beneficially reduces the influence of inter-symbol-interference with a complexity that is less than that of typical single carrier adaptive equalizers. OFDM has also been found to work well in multipath fading channels. These and other advantages render OFDM a strong candidate for use in future mobile communication systems, such as one being referred to as 4 G (fourth generation).
Under a frequency selective fading channel each sub-carrier of an OFDM modem is attenuated differently. The resultant sub-channel frequency functions are frequency-variant and may also be time-variant, hence adaptive modulation is often applied to an OFDM modem to improve the error performance and/or data throughout by assigning different transmission power and/or modulation and coding schemes to different sub-carriers. One aim is to achieve a good trade-off between throughput and error performance; another possible target is to maximize the net data throughput only, regardless of the resultant error performance. It is known to achieve performance goals by adjusting the sub-carriers' power and/or their modulation & coding scheme assignments. These are collectively known in the OFDM literature as adaptive OFDM, or bit/power loading.
Conventional OFDM modems employ inverse Fourier transforms in the transmitter to convert an OFDM symbol from the frequency domain to the time domain. Whereas the input to a processing block that performs the inverse Fourier transform is typically a (coded or uncoded) modulated symbol that is actually in the time domain, it is a well understood convention in the communication arts to refer to a symbol input to an inverse Fourier transform block as a frequency domain symbol, and the output from that block as a time domain symbol since that output symbol is to be sent over a time channel. This description employs that same convention. In adaptive OFDM according to the prior art, bit and/or power loading of each sub-carrier is adjusted based on channel conditions measured at the recipient of the data and fed back to the sender, or estimated by the sender based on return signals received from the recipient over the various sub-carriers. By adaptively loading more bits into the sub-carriers that have a higher signal to noise ratio, for example, throughput (defined as [1−error rate]*data rate) can be increased as compared to loading all sub-carriers with the same number of bits, without using additional bandwidth.
As increasing volumes of data is passed over finite bandwidth, especially with users wirelessly moving audio and video files, what is needed in the art is a method and apparatus to increase data throughput and/or improve error performance over that finite bandwidth. While described below in the context of an OFDM communication system, the invention may be used to advantage in any frequency-division multiplexed communication systems including OFDM and MC-CDMA.